The blue coloration on these katana blades is produced through a controlled bluing or heat-coloring process applied to the polished steel surface after the blade is forged and ground. In chemical bluing, the steel reacts with a solution that forms a thin iron oxide layer - magnetite rather than the red rust that degrades steel - which absorbs light to produce blue and grey tones. Heat-induced bluing achieves a similar effect through precise temperature control during finishing. The result is both aesthetic and mildly protective, as the oxide layer offers a marginal barrier against surface oxidation. To preserve the finish, collectors should avoid prolonged contact with bare hands and apply a light coat of choji or camellia oil every few months.
